The present invention relates to new peroxide curable fluoroelastomers particularly suitable for manufacturing O-rings.
Various types of fluoroelastomers are known in the art, widely used in all those fields where superior elastic properties associated with high thermochemical stability are required. For a wide survey on such products see for instance xe2x80x9cUllmann""s Encyclopedia of Industrial Chemistryxe2x80x9d, vol. A-11, pag. 417-429 (1988, VCH Verlagsgesellschaft).
It is also known that for manufacturing sealing elements, particularly O-rings, it is necessary to use elastomers endowed with particularly low compression set values. In fact, O-ring sealing effectiveness is as better as the article, upon compression, is able to recover initial dimensions. Since the fluoroelastomers are used in a wide temperature range, compression set values should be low not only at low temperatures, but also at high temperatures. Compression set values (measured at 200xc2x0 C. for 70 hours, according to ASTM Standard D395, Method B) lower than 25% are generally required. More particularly, military specifications (MIL-R-83248B) asks for O-rings having a maximum compression set value of 20% (measured at 200xc2x0 C. for 70 hours as well).
Fluoroelastomers which can meet such requirements are those curable ionically, which need addition of curing agents (for instance polyhydroxy compounds, such as Bisphenol AF or Bisphenol A), of suitable accelerators (for instance ammonium, phosphonium or amino-phosphonium salts), and of divalent metal oxides and/or hydroxides (for instance MgO, Ca(OH)2). Elastomers of this type are described for instance in patent application EP-525,685. However, ionic curing shows some drawbacks, among which the fact that a post-curing treatment is needed, generally carried out at 200xc2x0-260xc2x0 C. for 12-24 hours, in order to complete curing, and to eliminate volatile by-products, so as to improve and stabilize mechanical and elastic properties. This implies a remarkable increase in processing times and costs and therefore strongly limits the possibilities of large scale production.
As described in U.S. Pat. No. 4,243,770, fluoroelastomers can be crosslinked also by means of peroxides. To such purpose it is necessary to carry out the polymerization in the presence of suitable iodinated chain transfer agents, which introduce into the macromolecules iodine atoms in terminal position: in the presence of radicals deriving from a peroxide said iodine atoms act as cure sites in consequence of homolitic breakage of the Cxe2x80x94I bonds. Fluoroelastomers of this type generally do not require long post-curing treatments: in some cases it is sufficient a post-curing in air at about 200xc2x0-230xc2x0 C. for 1-4 hours. However, such products do not meet the specifications indicated above for O-ring manufacturing: the compression set value is indeed usually high, at least equal to 28-30% or higher.
The Applicant has now found a new class of fluoroelastomers as defined hereinunder, which, upon peroxide curing, need extremely short post-curing treatments (around 30-60 minutes at 180xc2x0-230xc2x0 C.) and are characterized by compression set values lower than 25% (measured on O-rings at 200xc2x0 C. for 70 hours according to ASTM Standard D395 Method B).
Therefore, a first object of the present invention are peroxide curable fluoroelastomers, having iodine atoms in terminal position, and monomeric units in the chain deriving from an iodinated olefin of formula:
CHRxe2x95x90CHxe2x80x94Zxe2x80x94CH2CHRxe2x80x94Ixe2x80x83xe2x80x83(I)
wherein: R is xe2x80x94H or xe2x80x94CH3; Z is a C1-C18 (per)fluoroalkylene radical, linear or branched, optionally containing one or more ether oxygen atoms, or a (per)fluoropolyoxyalkylene radical.
Further objects of the present invention are the iodinated olefins of formula (I), and the preparation process thereof, as described hereinunder.
As regards formula (I), Z is preferably a C4-C12 perfluoroalkylene radical, or a (per)fluoropolyoxyalkylene radical of formula:
xe2x80x94(Q)pxe2x80x94CF2Oxe2x80x94(CF2CF2O)m(CF2O)nxe2x80x94CF2xe2x80x94(Q)pxe2x80x94xe2x80x83xe2x80x83(II)
wherein: Q is a C1-C6, preferably C1-C3, alkylene or oxyalkylene radical; p is 0 or 1; m and n are numbers such that the m/n ratio is from 0.2 to 5 and the molecular weight of said (per)fluoropolyoxyalkylene radical is from 400 to 10,000, preferably from 500 to 1,000. Preferably, Q is selected from: xe2x80x94CH2Oxe2x80x94; xe2x80x94CH2OCH2xe2x80x94; xe2x80x94CH2xe2x80x94; xe2x80x94CH2CH2xe2x80x94.
The olefins of formula (I) can be prepared starting from compounds of formula Ixe2x80x94Zxe2x80x94I according to the following process:
(1) adding ethylene or propylene to a compound of formula Ixe2x80x94Zxe2x80x94I, thus obtaining a diiodinated product of formula:
Ixe2x80x94CHRxe2x80x94CH2xe2x80x94Zxe2x80x94CH2xe2x80x94CHRxe2x80x94Ixe2x80x83xe2x80x83(III)
where R and Z are defined as above;
(2) partially dehydroiodinating the product of formula (III) with a base (for instance NaOH, KOH, tertiary amines, etc.), so as to obtain the iodinated olefin of formula (I).
As to step (1), the addition of ethylene or propylene is usually carried out in the presence of suitable catalysts, such as redox systems, for instance CuI or FeCl3, in solution in an organic solvent, for instance acetonitrile. The addition reaction between a perfluoroalkyliodide and an olefin is described, for instance, by M. Hudliky in xe2x80x9cChemistry of Organic Fluorine Compoundsxe2x80x9d (2nd Edition, Ellis Horwood Ltd., Chichester, 1976), and by R. E. Banks in xe2x80x9cOrganofluorine Chemicals and Their Industrial Applicationsxe2x80x9d (Ellis Horwood Ltd. Chichester, 1979), or in J. Fluorine Chemistry, 49 (1990), 1-20 and in J. Fluorine Chemistry, 58 (1992), 1-8.
The dehydroiodination reaction of step (2) can be carried out either without any solvent, or by dissolving the diiodinated product in a suitable solvent (for instance a glycol such as diethylenglycol, or a long chain alcohol). To maximize the iodinated olefin yield, avoiding as far as possible a further dehydroiodination reaction with formation of the corresponding bis-olefin of formula CHRxe2x95x90CHxe2x80x94Zxe2x80x94CHxe2x95x90CHR, it is possible:
(a) to employ the base in defect with respect to the stoichiometric amount, with a molar ratio base/diiodinated compound preferably from 1.5 to 0.5, and then separating the iodinated olefin from the bis-olefin by fractional distillation; or
(b) to carry out the dehydroiodination reaction at reduced pressure, so as to remove she iodinated olefin from the reaction mixture as it forms, taking advantage of the fact that the latter has a boiling point lower than that of the starting diiodinated product; in such a case the reaction is preferably carried out without any solvent.
Alternatively, it is possible to carry out step (1) in defect of ethylene or propylene, to favour as much as possible formation of mono-addition product Ixe2x80x94Zxe2x80x94CH2xe2x80x94CHRxe2x80x94I (which can be separated from the di-addition product by fractional distillation); the mono-addition product is then dehydroiodinated as described above, with formation of olefin Ixe2x80x94Zxe2x80x94CHxe2x95x90CHR, which is finally submitted to a further addition of ethylene or propylene to give the iodinated olefin Ixe2x80x94CHRCH2xe2x80x94Zxe2x80x94CHxe2x95x90CHR.
When Z is a (per)fluoroalkylene radical, optionally containing one or more ether oxygen atoms, the starting diiodinated compound Ixe2x80x94Zxe2x80x94I can be obtained by telomerization of a (per)fluoroolefin C2-C4 or of a (per)fluorovinylether C3-C8 (for instance tetrafluoroethylene, perfluoropropene, vinylidene fluoride, perfluoromethylvinylether, perfluoro-propylvinylether, or mixtures thereof), by using a product of formula Ixe2x80x94(Rf)kxe2x80x94I (where k=0, 1; Rf=C1-C8 (per)fluoroalkylene radical) as telogenic agent. Telomerization reactions of this type are described, for instance, by C. Tonelli and V. Tortelli in J. Fluorine Chem., 47 (1990), 199, or also in EP-200,908.
When Z is a (per)fluoropolyoxyalkylene radical, the preparation of the products Ixe2x80x94Zxe2x80x94I is described, for instance, in U.S. Pat. No. 3,810,874.
The amount of monomeric units deriving from the iodinated olefin of formula (I) present in the fluoroelastomers object of the present invention is generally from 0.01 to 1.0 moles, preferably from 0.03 to 0.5 moles, even more preferably from 0.05 to 0.2 moles, per 100 moles of the other basic monomeric units.
The total iodine amount in the fluoroelastomers object of the present invention is, on the average, from 1.8 to 5.0, preferably from 2.0 to 4.0, iodine atoms per chain. The average number of iodine atoms per chain in terminal position is in turn generally from 1.0 to 2.0, preferably from 1.5 to 1.8. The iodine atoms in terminal position can be introduced, as described in U.S. Pat. No. 4,243,770, by adding during polymerization iodinated chain transfer agents, such as for instance compounds of formula RfTx, wherein Rf is a (per)fluoroalkyl or a (per)fluorochloroalkyl having from 1 to 8 carbon atoms, while x in 1 or 2. In particular, the iodinated chain transfer agent can be selected from: 1,3-diiodoperfluoropropane, 1,4-diiodoperfluorobutane, 1,6-diiodoperfluorohexane, 1,3-diiodo-2-chloroperfluoropropane, 1,2-di(iododifluoromethyl)-perfluorocyclobutane, monoiodoperfluoroethane, monoiodoperfluorobutane, 2-iodo-1-hydroperfluoroethane, etc. Particualrly preferred are diiodinated chain transfer agents (x=2). Alternatively, it is possible to use as chain transfer agents alkali or alkaline-earth metal iodides, as described in U.S. Pat. No. 5,173,553. The amount of chain transfer agent to be added to the reaction medium is chosen according to the molecular weight which is intended to be obtained and to the chain transfer agent effectiveness.
The basic structure of the fluoroelastomers object of the present invention can be selected in particular from:
(1) VDF-based copolymers, where VDF is copolymerized with at least a comonomer selected from: perfluoroolefins C2-C8, such as tetrafluoroethylene (TFE), hexafluoropropene (HFP); C2-C8 chloro- and/or bromo-fluoroolefins, such as chlorotrifluoroethylene (CTFE) and bromotrifluoro-ethylene; (per)fluoroalkylvinylethers (PAVE) CF2xe2x95x90CFORf, wherein Rf is a C1-C6 (per)fluoroalkyl, for instance trifluoromethyl, bromodifluoromethyl, pentafluoropropyl; (per)fluoro-oxyalkylvinylethers CF2xe2x95x90CFXO, where X is a C1-C12 perfluoro-oxyalkyl having one or more ether groups, for instance perfluoro-2-propoxy-propyl; C2-C8 non-fluorinated olefins (Ol), for instance ethylene and propylene; typical compositions are the following: (a) VDF 45-85%, HFP 15-45%, TFE 0-30%; (b) VDF 50-80%, PAVE 5-50%, TFE 0-20%; (c) VDF 20-30%, Ol 10-30%, HFP and/or PAVE 18-27%, TFE 10-30%;
(2) TFE-based copolymers, where TFE is copolymerized with at least a comonomer selected from: (per)fluoroalkylvinylethers (PAVE) CF2xe2x95x90CFORf, where Rf is defined as above; (per)fluoro-oxyalkylvinylethers CF2xe2x95x90CFOX, wherein X is defined as above; C2-C8 fluoroolefins containing hydrogen and/or chlorine and/or bromine atoms; C2-C8 non-fluorinated olefins (Ol); typical compositions are the following: (d) TFE 50-80%, PAVE 20-50%; (e) TFE 45-65%, Ol 20-55%, VDF 0-30%; (f) TFE 32-60%, Ol 10-40%, PAVE 20-40%; (g) TFE 33-75%, PAVE 15-45%, VDF 10-22%.
The preparation of the fluoroelastomers object of the present invention can be carried out by copolymerization of the monomers in aqueous emulsion according to methods well known in the art, in the presence of radical initiators (for instance alkali metal or ammonium persulphates, perphosphates, perborates or percarbonates), optionally in combination with ferrous, cuprous or silver salts or other easily oxidable metals. In the reaction medium are usually present also surfactants of various types, among which particularly preferred are the fluorinated surfactants of formula:
Rfxe2x80x94Xxe2x88x92M+
wherein Rf is a C5-C16 (per)fluoroalkyl chain or a (per)fluoropolyoxyalkylene chain, Xxe2x88x92 is xe2x80x94COOxe2x88x92 or xe2x80x94SO3xe2x88x92, M+ is selected from: H+, NH4+, alkali metal ion. Among the most commonly used, we can cite: ammonium perfluorooctanoate, (per)fluoropolyoxyalkylenes terminated with one or more carboxyl groups, etc.
In a preferred embodiment, the fluoroelastomers object of the present invention are prepared in the presence of an aqueous microemulsion of perfluoropolyoxyalkylenes, as described in U.S. Pat. No. 4,864,006, or in the presence of an aqueous microemulsion of fluoropolyoxyalkylenes having hydrogenated end groups and/or hydrogenated repetitive units, as described in EP-625,526.
The amount of iodinated olefin of formula (I) to be added to the reaction mixture depends on the amount of units deriving therefrom which are intended to be obtained in the final product, bearing in mind that, at the low amounts employed according to the purposes of the present invention, practically the whole amount of the iodinated olefin present in the reaction medium enters the chain.
The polymerization reaction is generally carried out at a temperature of from 25xc2x0 to 150xc2x0 C., at a pressure up to 10 MPa.
When polymerization is completed, the fluoroelastomer is isolated from the emulsion by means of conventional methods, such as coagulation by addition of electrolytes or by cooling.
The peroxide curing is carried out, according to the art, by addition of a suitable peroxide capable of generating radicals by heating. Among the most commonly used we car cite: dialkylperoxides, such as for instance di-tertbutyl-peroxide and 2,5-dimethyl-2,5-di(tertbutylperoxy)hexane; dicumyl peroxide; dibenzoyl peroxide; ditertbutyl perbenzoate; di[1,3-dimethyl-3-(tertbutyl-peroxy)butyl]carbonate. Other peroxide systems are described, for instance, in EP Patents 136,596 and 410,351.
To the cure mixture other products are then added, such as:
(a) curing coagents, in an amount generally from 0.5 to 10%, preferably from 1 to 7%; by weight with respect to the polymer; among them commonly used are: triallyl-cyanurate; triallyl-isocyanurate (TAIC); tris(diallyl-amine)-s-triazine; triallylphosphite; N,N-diallyl-acrylamide; N,N,Nxe2x80x2,Nxe2x80x2-tetraallyl-malonamide; trivinyl-isocyanurate; 2,4,6-trivinyl-methyltrisiloxane, etc.; TAIC is particularly preferred.
(b) a metal compound, in an amount of from 1 to 15%, preferably from 2 to 10%, by weight with respect to the polymer, selected from divalent metal oxides or hydroxides, such as for instance Mg, Zn, Ca or Pb, optionally associated to a weak acid salt, such as for instance Ba, Na, K, Pb, Ca stearates, benzoates, carbonates, oxalates, or phosphites;
(c) other conventional additives, such as fillers, pigments, antioxidants, stabilizers, and the like.
As said above, the fluoroelastomers object of the present invention do not require long post-curing treatments; to remove possible volatile by-products, after curing it is sufficient to treat the product in air for 30-60 minutes at 180xc2x0-230xc2x0 C. As demonstrated by the experimentation carried out by the Applicant, longer post-curing periods do not lead to any significant improvement of mechanical and/or elastic properties of the product. This allows to drastically reduce processing times, with consequent increase of productivity for molding processes on an industrial scale.
The present invention will be now better illustrated by the following working examples, whose purpose is merely indicative but not limitative of the scope of the invention itself.